1. Field of the Invention
This invention relates to a bobbin thread end holding device in a sewing machine, and more particularly to a bobbin thread end holding device in a sewing machine which employs a so-called DP type half-turn shuttle hook, and which does not use the thread hole of the engaging member of a bobbin casing, or uses a bobbin casing whose engaging member has no thread hole.
2. Related Art
A full-turn shuttle hook and a half-turn shuttle hook are available as shuttle hook bodies in sewing machines. An example of the half-turn shuttle hook is such that the needle comes between an upper thread loop and the bobbin; more specifically, the needle drop position is between the bobbin thread lead-out point of the bobbin and the sharp protrusion of the shuttle hook body (hereinafter referred to as "a DBS shuttle hook", when applicable). Another example of the half-turn shuttle hook is such that the upper thread loop comes between the needle and the bobbin; more specifically, the sharp protrusion of the shuttle hook body is between the needle drop position and the bobbin thread lead-out point of the bobbin (hereinafter referred to as "a DPS shuttle hook", when applicable).
A sewing machine having the DBS shuttle hook forms perfect stitches in a forward feeding operation (in which a fabric is fed in a right-to-left direction as viewed from the operator), and forms hitch stitches in a reverse feeding operation (in which a fabric is fed in a left-to-right direction as viewed from the operator). Hence, the sewing machine of this type is extensively employed for a lock stitching operation.
In general, in a lock stitching operation, perfect stitching should be employed to form straight stitches from the view point of the quality of stitches, and in order to eliminate the difficulty that a thread gets loose at both ends of a line of stitches, the latter should be ended by hitch stitching. In order to meet those requirements, the DBS shuttle hook is employed.
However, a sewing machine having the DBS shuttle hook is not suitable for an omnidirectional stitching operation.
A sewing machine having the DPS shuttle hook is also known in the art, see Japan Patent No. 99353 for instance. The sewing machine is able to form perfect stitches both in the forward feeding operation and in the reverse feeding operation, thus being suitable for an omnidirectional stitching operation; however, it is not popularly employed in the art yet.
However, it should be noted that the DPS shuttle hook responds well to the variation in thickness of a fabric, thus being suitable for sewing a heavy weight fabric and for use of a thread large in yarn number count.
FIG. 12 shows a conventional bobbin casing 100 used with an ordinary half-turn shuttle hook. As shown in FIG. 12, the bobbin casing 100 has an engaging member 101, in which a thread hole 102 is formed.
A bobbin thread 104 supplied from a bobbin 103 accommodated in the bobbin casing 100 is passed through the thread hole 102 of the engaging member 101 of the bobbin case 100.
The bobbin case is set in the sewing machine in such a manner that it faces forward so that it is convenient for the operator to handle when it is set in the sewing machine or removed from the latter.
The direction of rotation of the shuttle hook body should be so determined that the upper thread on the needle side is twistable because a sewing thread for a sewing machine is fundamentally of Z-twist. That is, in the case of the DBS shuttle hook, the direction of rotation of the shuttle hook body is clockwise as viewed from the operator side; and in the case of the DPS shuttle hook, the direction of rotation of the shuttle hook body is counterclockwise.
FIG. 13 shows the fundamental configuration of a thread control hole in a sewing machine employing the DBS shuttle hook. A thread control board, namely, a shuttle hook race cap 105 has the thread control hole 106.
As shown in FIG. 14, a bobbin thread 104, a cloth-side upper thread 114C, and a needle-side upper thread 114N are positioned in the thread control hole 106. A movable knife 107 having a supporting shaft 108 on the right side of the needle drop point O as viewed from the operator is held in front of the thread control hole. In a thread displacing operation (described later), the movable knife operates to displace the bobbin thread 104 and the upper thread in the stated order.
In a thread cutting device, as shown in FIG. 14 the bobbin thread 104 and the cloth-side upper thread 114C which are determined to be cut by the construction of the DBS shuttle hook are substantially in alignment with the needle drop point O, so that the amounts of displacement of the bobbin thread 104 and the cloth-side upper thread 114C which are due to the forward movement of the movable knife 107 are minimized.
The threads are cut as follows: The movable knife 107 is moved forwardly to start a thread displacing operation (in which, before cut, the bobbin thread 104 and the cloth-side upper thread 114C are led to a thread drawing portion 107a of the movable knife 107 by the forward movement of the latter 107). In this operation, since the upper thread 114 has been sufficiently drawn into the shuttle hook, the tip end 107c of a thread displacing portion 107b of the movable knife 107 is positively placed between the cloth-side upper thread 114C and the needle-side upper thread 114N.
And the bobbin thread 104 is displaced by the thread displacing portion 107b. The amount of displacement of the bobbin thread 104 is the difference between the distance between the front tip end 107c of the thread displacing portion 107b and the supporting shaft 108 and the distance between the rear tip end 107d of the thread displacing portion 107b. In this case, no great forces other than the restoring force of the bobbin thread and the fabric feeding force of the sewing machine are applied to the bobbin thread 104 which has been bent being displaced; however, since the amount of displacement is small, the bobbin thread 104 is restored sufficiently.
Next, the cloth-side upper thread 114C is displaced by the thread displacing portion 107b. When compared with the bobbin thread 104, the upper thread 114 is greatly affected in behavior, for instance, by the pull-up operation of the thread take-up lever and the thread take-in operation of the shuttle hook, so that the cloth-side upper thread 114C is moved in the direction of the arrow in FIG. 14. In this case, since the direction of the arrow is coincident with the direction of forward movement of the movable knife 107, the amount of displacement of the cloth-side upper thread 114C is minimized.
Thereafter, the movable knife 107 is retracted maximumly, so that the cloth-side upper thread 114C thus displaced moves over the thread displacing portion 107b of the movable knife 107 to reach the thread drawing portion 107a. Next, the movable knife 107 is returned, so that the cloth-side upper thread 114C and the bobbin thread 104 are caught by the thread drawing portion 107a in the stated order, and the upper thread 114 is pulled upwardly by the thread take-up lever, and its part is held as wound around the movable knife 107.
Under this condition, the bobbin thread 104, being pulled out by the movable knife 107, is let out as much as required. If, in this operation, the bobbin thread is pulled out too fast, then the bobbin is excessively turned, which adversely affects the following sewing operation. In order to eliminate this difficulty, the speed of return of the movable knife 107 is suitably controlled. After cut, the part of the upper thread 114 which is held by the movable knife 107 is a remaining thread on the side of the needle.
Next, the cloth-side upper thread 114C and the bobbin thread 104 are cut with the thread cutting edge 107e (the edge of the front end of a so-called "eye") of the movable knife and a stationary knife. The cutting position is just beside the needle hole. Hence, the thread left on the rear side of the fabric is short, so that the resultant stitches are high in quality.
Thus, by setting the width of the movable knife 107 to a suitable value, and by determining the configuration of the thread control hole 106 of the shuttle hook race cap 105 and the thread cutting direction in the above-described manner, it is possible to make the end portions of the needle-side upper thread and the bobbin thread long enough to start the following sewing operation satisfactorily.
A sewing machine employing the DBS shuttle hook and a sewing machine employing the DPS shuttle hook may be compared with each other as follows:
FIGS. 15A and 15B show comparison between those sewing machines. In FIG. 15A, reference numeral 111 designates the DBS shuttle hook; 112, the sharp protrusion of the latter 111; 113, the needle; 114, the upper thread; and R, the upper thread loop. In FIG. 15B, reference numeral 115 designates the DPS shuttle hook; and 116, the sharp protrusion of the latter 115.
In the case where the DBS shuttle hook 111 is employed, the distance (1) between the needle drop point and the bobbin thread lead-out point is as shown in FIG. 5A; while in the case where the DPS shuttle hook 115 is employed, the distance (2) between the needle drop point and the bobbin thread lead-out point is as shown in FIG. 15B. That is, the distance (2) is larger than the distance (1). This is an important difference between the two cases.
That is, fundamentally, in a sewing machine employing the DPS shuttle hook 115, the distance (2) between the needle drop point and the bobbin thread lead-out point is shifted laterally (offset) as much as (the width of the lace of the shuttle hook body +.alpha. (alpha)) when compared with the distance (1) between the needle drop point and the bobbin thread lead-out point in the sewing machine employing the DBS shuttle hook 111.
As a result, the force of the thread take-up lever (not shown) to pull up the upper thread 114 is markedly decreased. This is a fundamental factor which adversely affects the tightening of the thread.
Incidentally, when the needle 113 is held not to pass the fabric, the latter is fed, and at the same time the thread take-up lever operates to tighten the thread.
FIGS. 16A and 16B show states of the shuttle hook bodies with a fabric feeding mechanism in operation. More specifically, FIG. 16A shows the engaging member 101 which stops the rotation of the bobbin casing in the sewing machine using the DBS shuttle hook; and FIG. 16B shows the engaging member 101 which stops the rotation of the bobbin casing in the sewing machine using the DPS shuttle hook. In FIG. 16A, reference numeral 121 designates a shuttle hook race ring; 122, an engaging groove; 123, a throat plate; and 124, a needle hole (needle passing hole). Further in FIG. 16A, reference character A designates an arrow indicating a fabric feeding direction; and CW, an arrow indicating the direction of rotation of the DBS shuttle hook 111 (clockwise direction). In FIG. 16B, reference character B designates an arrow indicating a fabric feeding direction; and CCW, an arrow indicating the direction of rotation of the DPS shuttle hook (counterclockwise direction).
FIGS. 17A and 17B are enlarged diagrams showing essential components shown in FIGS. 16A and 16B, respectively.
The bobbin thread 104 is pulled by the fabric feeding force as indicated by the arrows A and B in FIGS. 16A and 16B. As a result, in the case of the DBS shuttle hook 111, the bobbin casing 100 is turned in the direction of the arrow CW (clockwise) as shown in FIG. 16A; and in the case of the DPS shuttle hook 115, the bobbin casing 100 is turned in the direction of the arrow CCW (counterclockwise) as shown in FIG. 16B.
As a result, in the case of the DBS shuttle hook 111, as shown in FIG. 17A the engaging member 101 is pushed to the right against the engaging groove 122 of the shuttle hook race ring 121 which is able to stop the rotation of the bobbin casing 100; and in the case of the DPS shuttle hook 115, as shown in FIG. 17B the engaging member 101 is pushed to the left against the engaging groove 122 of the shuttle hook race ring 121.
In this connection, as for the upper thread 114, a movable side (the side of the needle 113) and a stationary side (the side of the fabric) should be taken into consideration. In the case of the DBS shuttle hook 111, as shown in FIG. 17A the loop R of the upper thread on the movable side (the side of the needle 113) to which the pull-up force of the thread take-up lever is readily transmitted enters the space between the engaging member 101 and the right wall surface of the engaging groove 122.
As is apparent from comparison of FIG. 15A and 15B, in the case of the DBS shuttle hook 111, the angle of entrance of the loop R is such that the latter is much less in the amount of offset than in the case of the DPS shuttle hook 115. And the loop is pulled upwardly with respect to the engaging member 101, which readily permits the passage of the upper thread 114 as shown in FIG. 18.
On the other hand, in the case of the DPS shuttle hook 115, as shown in FIG. 17B the loop R of the upper thread 114 which enters the space between the engaging member 101 and the left wall surface of the engaging groove 122 comes on the stationary side (on the side of the fabric), and therefore the force of the thread take-up lever is scarcely transmitted to it, and the loop is larger in the amount of offset than in the case of the DBS shuttle hook 111. In addition to this disadvantage, the upper thread 114 turns around the engaging member 101 from above, and therefore it becomes rather difficult to leave from the engaging member.
Those difficulties depend on the fabric feeding direction and on the stitching pitch, and are most significantly encountered when the stitching pitch is large and the fabric is fed in a right-to-left direction.
Mainly because of those difficulties, the thread is unevenly tightened.
When leaving from the DPS shuttle hook 115, the upper thread 114, depending on the timing of entrance of a driver adapted to drive the shuttle hook body, is generally shaped as shown in FIGS. 19A and 19B. In FIGS. 19A and 19B, reference numeral 131 designates the driver.
In this case, the thickness of the inner side portion of the driver 131 and its curvature (radius), and the width of the right and left flanges of the thread disengaging section must be taken into account. If the driver 131 is made thin, then the upper thread 114 may be tightened well, and its resistance is decreased. However, the modification of the driver 131 in the above-described manner gives rise to another problem. That is, in the case where the thread is used which is high in twist movement or is liable to run (vibrate) being high in tension, the thread itself is twisted, so that it is unevenly tightened or forms unwanted loops.
On the other hand, it is important for operational balance that the time instant that the upper thread disengages from the driver 131 occurs slightly later than the time instant that the upper thread disengages from the engaging member 101 of the bobbin casing 100. This fact eliminates difficulties that the thread is unintentionally twisted, and it forms unwanted loops or knots.
In the case of the DPS shuttle hook 115, because of the difficulties described with reference to FIG. 17B, the thread 114 is increased in resistance being pushed as indicated by the arrow (*), and the timing that the thread leaves from the engaging member is delayed with respect to the timing that the thread leaves from the driver 131, as a result of which the upper thread 114 remains over the engaging member 101 of the bobbin casing 100.
As a result, the thread 114 caught by the engaging member 101 of a thread disengaging section which is larger in the amount of offset than in the case of the DBS shuttle hook, is pulled sideward, thus forming unwanted loops and knots.
In this connection, it may be thought of that, if the upper thread is smoothly disengaged from the engaging member of a bobbin casing whose thread disengaging portion is made larger in the amount of offset than in a sewing machine using the DBS shuttle hook so that it is applicable to a sewing machine using the DPS shuttle hook, thus forming no unwanted loops or knots, then the resultant stitches will be fine in quality.
That is, the use of the thread hole of the engaging member of the bobbin casing should be omitted, to improve the tightening of the thread in quality. In addition, a bobbin casing should be formed which has no thread hole.
Referring back to FIG. 12, the bobbin thread 104 supplied from the bobbin 103 in the bobbin casing 100 of the DBS shuttle hook is passed through the thread hole 102 of the engaging member 101 of the bobbin casing 100. Accordingly, as shown in FIGS. 13 and 14, the end portion of the bobbin thread 104 which has been cut and located in the thread control hole 106 of the shuttle hook race cap is held stable.
However, in the case where the bobbin casing is applied to the DPS shuttle hook with the use of the thread hole of its engaging member of the bobbin casing omitted in view of the quality of the tightening of the thread, an ill effect is produced; that is, the end portion of the bobbin thread which has been cut in the thread control hole of the shuttle hook race cap is unstable.